Collections allow you to curate illustrations, manga, novels, and Ugoira posted on pixiv, links to other pixiv services (e.g., pixiv Encyclopedia, BOOTH, pixivFANBOX), external links, and text, and compile them on a single page.
At present, the feature is only available on the desktop and mobile versions of pixiv.
You cannot view or post collections from the pixiv app.
However, you can still use the app to prevent your works from being added to other people's collections.
To learn more, see the following page.